Many M2+M4+F6 have cation ordered ReO3-type structures, several of which are cubic at room temperature. The cubic ReO3-type structure is often used to illustrate how negative thermal expansion (NTE) can arise from vibrational motion, even though ReO3 displays positive thermal expansion (PTE) at room temperature. We have previously shown that ScF3 and CaZrF6, both with cubic ReO3-type structures, display very strong negative thermal expansion over a wide temperature range (volume CTEs of ~-24 and -36 ppmK-1 at 300K). The key to this behavior is maintaining cubic symmetry to very low temperatures, which is presumably related to the highly ionic bonding in these materials. Most MF3 undergo a symmetry lowering phase transition on cooling from high temperature and display PTE below the transition, but ScF3 and CaZrF6 do not show this transition &gt; 10 K. We have also shown CaZrF6 to have a bulk modulus of 37 GPa, and begin to disorder at 0.4 GPa. We have begun to examine the properties of M2+M4+F6 to establish the chemical factors that control the phase transitions, negative thermal expansion, and pressure behavior in these materials. CaNbF6 and MgZrF6, were chosen for examination as they were previously reported to be cubic at room temperature, but their behavior with change in temperature or pressure were completely unknown. The thermal expansion of CaNbF6 and MgZrF6 were investigated initially using variable temperature, 100-500 K, X-ray diffraction. CaNbF6 and MgZrF6 both remained cubic and showed NTE over the temperature range of 100-500 K with a volume CTE of -41 ppmK-1, and -10 ppmK-1 at 300 K, respectivly. Low temperature neutron diffraction studies were done, and CaNbF6 stayed in the cubic phase down to 10 K, and showed a minimum volumetric CTE of -65.8 ppmK-1 at 70 K. MgZrF6 remained cubic down to 50 K and then underwent a symmetry lowering phase transition. While in the cubic phase, MgZrF6 reaches a volumetric CTE of -22 ppmK-1 at 120 K. The lower symmetry phase shows PTE from 10-50 K. Variable pressure X-ray diffraction data were also collected. CaNbF6 has a bulk modulus of 35.2 GPa and displays a pressure induced softening for the cubic phase, which persists until a pressure of 0.37 GPa. The material then undergoes a symmetry lowering phase transition, and amorphizes at 3.95 GPa. MgZrF6 has a bulk modulus of 44.17 GPa while in the cubic phase. The material undergoes a two crystal to crystal phase transitions at 0.17 GPa, and 1.12 GPa respectively.

Comparison of the negative thermal expansion and behavior on compression for CaZrF6, CaNbF6 and MgZrF6

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.